Will a wife incur sin or blame for informing her husband, who wishes to marry a second time, that she will seek a divorce if she cannot tolerate it?

You are not at fault for what you said to your husband, because if a woman is clearly harmed by remaining married to her husband, she is permitted to request a divorce. Islam legislated polygyny for great benefits, and a wife should assist her husband in this endeavor if possible. It is a condition for one who marries more than one woman to be just among them: "marry such women as seem good to you, two, three, or four; but if you fear that you will not be able to deal justly with them, then only one."